(1) A method whereby the analog signal and the digital signal are adjusted and attenuated by separate attenuating means.
(2) A method whereby a level of the digital signal is attenuated and adjusted by setting an attenuation coefficient of the digital signal through fixing step width discretely (for example, by making an output change by 0.5 dB every time 1 pulse is counted in an analog electronic attenuator) in order to make an attenuated value of the digital signal correspond to an attenuated value of the analog signal.
(3) A method whereby a level of the digital signal is attenuated and adjusted by means for detecting an operation rotation angle or slide width of an analog attenuator, the attenuation coefficient of the digital signal being set based on a value detected by the above means.
However, the attenuation methods (1) to (3) described above present the following problems.
In the method (1), signals belonging to two systems, i.e., the analog system and the digital system, must be adjusted separately. Since an attenuation adjustment circuit becomes necessary for each of the systems, it becomes complicated to adjust the attenuation adjustment circuits resulting in deterioration in operability. Another problem is an overall rise in cost.
In the method (2), fine adjustment cannot be carried even with respect to the analog signal out within a step width smaller than the fixed step width. A special circuit element such as the analog electronic attenuator further becomes necessary.
In the method (3), relatively bulky detection means such as a rotary encoder becomes necessary for detecting the operation rotation angle or the slide width of the analog attenuator. A further problem is that it is difficult to make the attenuating factor of the signal of the analog system (referred to hereinafter as analog system signal) exactly identical to the attenuating factor of the signal of the digital system (referred to hereinafter as digital system signal).